1. Field of the Invention
The present invention relates to the production of a transparent plastic sheet of high optical quality having at least a layer exhibiting scoring resistance and abrasion resistance.
2. Description of the Background
Laminated safety glasses, used particularly as vehicle windshields, having a transparent rigid support of silicate glass, a plastic layer having energy-absorbing properties (EA layer) and a coating plastic layer having surface properties particularly of resistance to scoring or scratching and abrasion, called the inside protective layer (IP layer), have been described in French patents 2,134,255, 2,398,606, and also in European patent 0,054,491. The coating layer having the surface properties used in these laminated glasses is, for example, that described in French patents 2,187,719 and 2,251,608. This layer, referred to as self-healing, from which surface impressions spontaneously disappear after a short lapse of time, has, under normal temperature conditions, a high elastic deformation capacity, a slight modulus of elasticity, less than 2000 daN/cm2, and preferably less than 200 daN/cm2, and an elongation at break of more than 60% with less than 2% plastic deformation and preferably an elongation at break of more than 100% with less than 1% plastic deformation. Preferred layers of this type are thermosetting polyurethanes having a modulus of elasticity of about 25 to 200 daN/cm2 and an elongation of about 100 to 200% with less than 1% plastic deformation.
To produce the laminated glasses described above, a two-layer sheet is generally previously prepared by first, forming a thermosetting polyurethane layer by casting of the reaction mixture of the components on a casting support, and then polymerizing the monomers and forming a thermosetting layer with a thickness that can vary from 0.1 to 0.8 mm and forming on the first layer the second layer having energy-absorbing properties by casting of the reaction mixture of components or also by extrusion of a suitable resin that is already polymerized.
The reactive layer without solvent used to produce the thermosetting polyurethane layer requires, in order obtain a high optical quality, the production of a layer with a thickness greater than 0.1 Mm and preferably greater than 0.2 mm.
It has been found that a thinner layer of the order of some dozens of microns could still attain the required surface properties such as resistance to scoring and resistance to abrasion when this layer is used to coat another soft layer, as is the case for the laminated glasses described above where said layer with the surface properties is used to coat the EA layer.
Moreover,,the production of such a thin layer would seem to be best effected by spraying with a spray gun inasmuch as this technique can deposit small amounts of deposited material. Unfortunately, spraying with a spray gun to form a thin layer is not a satisfactory technique as layers so produced are not of homogeneous thickness nor is the optical quality good. In particular, the optical quality is found to suffer from orange peel. This is believed to be due to the viscosity differential between the components forming the mixture to be sprayed. Although some attempts have been made to alleviate such drawbacks by using particular solvents or mixtures of solvents. However, these attempts have proven unsuccessful inasmuch as other problems arise from solvent evaporation.
Thus, a need continues to exist for a process for the continuous production of a plastic sheet which can be used in laminated glasses, said sheet having at least a thin layer of high optical quality and also having excellent resistance to scoring and abrasion.